As discussed in The Events Recorded in the Bible, the question of whether Amestris and Vashti are one and the same is the reason for most of the disagreement about who the king featured in Esther really is. I think it’s a safe assumption, so here I’ll go through a bit about what we know of Amestris, what we know of Vashti, and how I meshed the two for one comprehensive character.
First, Vashti—she’s mentioned only in that first chapter of Esther, and all it says is that she’s very beautiful. So beautiful that the king commands her to leave her feast for the women of the court and come into his feast for the men, wearing her crown, so that all might see her beauty. Some assume he ordered her to come in wearing only her crown, but there’s absolutely nothing to back this up. The wording of the Bible does not in any way imply this. She refuses, which enrages her husband. Upon the advice of his council, he issues a proclamation saying that Vashti might 
never come into his presence again, so that all the women of the kingdom won’t see her disobedience and think they can act the same.
I suppose people insert the “only” to try to make sense of why Vashti would refuse the request. Well, for starters, it went against their cultural traditions to call a hostess away from her feast before a group of only men. For another, if she was Amestris, she would have been about 8 months pregnant with Artaxerxes at the time. Plus, of course, I add some extra reasoning too. =)
Now, Amestris. When I first sat down to this story and was getting started on research, I had this vague idea that I wanted to do something unexpected, so maybe I’d have Kasia be friends with Vashti. But upon researching Amestris (and assuming her the same as Vashti) I realized that wasn’t going to happen. Amestris—Amestris is just nasty. Evil. Horrified the ancient world regularly, and though the Greeks chalked it up to how terrible the entire Persian culture was, historians now chalk it up to a private illness. The woman was seriously unhinged.
A few examples. She had her sister-in-law mutilated (as detailed in The Affair). She was notoriously jealous of the young, beautiful women her husband brought into the palace, given that she was (gasp) in her thirties by the time she became queen. She purportedly had children, both boys and girls, buried alive to guarantee her entrance into the afterlife (though human sacrifice was NOT a part of Persian religion). Nice she wasn’t.
So after the big affair and its fallout, I’m guessing relations between her and Xerxes were, well, worse than ever. Hence why, when we hear that Xerxes’ assassination resulted from a conspiracy rooted in the harem, I read it as being her idea. Is this fact? No. Plausible? Very.
When her youngest son took the crown, Amestris took on the role of Queen Mother. Assuming she was the deposed Vashti, this would be when she got her ultimate revenge and the missing tendrils of her power back. As Queen Mother, she continued to horrify the world with her rages.
In Jewel of Persia, Kasia claims that Vashti carries the same meaning in Hebrew as Amestris, and Xerxes jokes that they ought to call her that out of spite, since she would hate it. =)
(Photo of the Apadana palace in Persepolis)